Stress of climate change killing forests, new studies show

A forest fire burns in the Canary Islands on August 13.New ecological studies being published lately all point to the same basic conclusion: the stress of climate change is killing the world's forests.

One study, released by a team at the University of Colorado Boulder, reveals that mid-elevation forests (those at altitudes roughly between 2,000 to 2,500 metres) represent an ecological "tipping point" between the more moisture-dependent forests at lower elevations and the more sunlight and temperature-dependent forests at higher elevations.

Mid-elevation forests depend the most on the annual snow-pack that accumulates at those altitudes, and are therefore the most vulnerable to climate change. Researchers used satellite measurements of 'forest greenness' and measurements of snow depth from ground-based instruments. They found that as global temperatures rise, there is a reduction in the depth of the snow-pack, and this leaves these mid-altitude forests more vulnerable to forest fires, insect infestations, and tree-species die-offs.

"Our results provide the first direct observations of the snowpack-forest connections across broad spatial scales," said Noah Molotch, a research scientist at CU-Boulder's Institute of Arctic and Alpine Research. "Finding the tipping point between water-limited forests and energy-limited forests defines for us the region of the greatest sensitivity to climate change -- the mid-elevation forests -- which is where we should focus future research."

In another recent study, scientists pooled their knowledge of drought-induced tree species die-offs. While drought conditions and how they factored into the issue of forest trauma had been studied in the past, they hadn't been studied on their own until now. Tree species loss due to droughts can fundamentally change a forest by altering how much sunlight reaches the forest floor, changing the availability of moisture and soil nutrients, and increasing both soil erosion and the forest's vulnerability to fire.

"This is the first snapshot of how these things fit together," said William Anderegg, a doctoral candidate in Stanford University's Department of Biology, and lead author of the study. "So we don't have a lot of final answers yet."

Tree deaths have a double effect the carbon cycle. If a tree dies, it no longer absorbs carbon dioxide as part of the process of photosynthesis, and as the tree decays it frees up trapped carbon that once again enters the carbon cycle.

"This whole area has been fairly under-studied until now," Anderegg said. "We need more research with a really wide net."

Anderegg and his colleagues have started a collaborative website to better share their findings on drought and tree deaths, and better coordinate their research efforts.